1. Field of the Invention
The present invention relates to a flat acoustic conversion device, and particularly to a flat acoustic conversion device such as a flat speaker which can be used as a flat speaker, a flat microphone, and a microphone.
2. Description of the Related Art
FIG. 1 is a diagram showing the basic structure of a conventional flat speaker. This flat speaker is provided with a plurality of bar magnets 1 which are arranged in parallel on a yoke 4, a vibrating membrane 2 disposed parallel and adjacent to a magnetic pole face of the bar magnets 1, and a plurality of coils 3 each formed at a position on the vibrating membrane which corresponds to the magnetic pole face of the bar magnets in such a manner that the electric current flows in a direction orthogonal to the magnetic field generated by the bar magnets 1. Each coil 3 is disposed at a position where the greater part of the inner periphery of each coil faces the magnetic pole face of a bar magnet and the remaining part is disposed outside a position corresponding to the outer edge of the bar magnets. The outer edge of the vibrating membrane is fixed by a fixing member in such a manner that the vibrating membrane is able to vibrate together with the coils. Because the current flowing through each coil 3 is affected by the force from the magnetic field of the bar magnets in accordance with Fleming's left-hand law when alternating current is fed to each coil 3, the vibrating membrane 2 is vibrated in a direction perpendicular to the surface of the vibrating membrane together with the energized coils which enables an electric signal to be converted into an acoustic signal.
Further, by vibrating the vibrating membrane 2 in a direction orthogonal to the surface of the vibrating membrane and converting an acoustic signal into an electric signal in accordance with Fleming's right-hand law, the device is able to be used as a microphone.
However, in the above conventional flat speaker, because a large portion of the coil is positioned facing the magnetic pole face of the bar magnets, a magnetic field acts on the coil portion positioned facing the magnetic pole face of the bar magnets in a direction perpendicular to the surface of the vibrating membrane. Therefore, force, which acts on the electric current flowing through the coil portion from the electric field, acts in a direction along the surface of the vibrating membrane. The problem then arises that this force in a direction along the surface of the vibrating membrane causes twisting in the surface of the vibrating membrane and becomes a noise component in the acoustic signal leading to a reduction in the sound quality.
Moreover, because the plurality of bar magnets are arranged in parallel longitudinally, the length of the portion of the coil which interlinks with the magnetic field is approximately twice the product obtained by multiplying the length of the bar magnets by the number of windings of the coil. Consequently, the proportion of the surface area of the vibrating membrane occupied by those portions of the coils interlinking with the magnetic field is low. Therefore, the problem has been that, not only has it not been possible to obtain sufficient volume due the deterioration in the efficiency of the acoustic conversion, but the sound quality has also been low.
Furthermore, the shape of the speakers has been determined by the length and number of the bar magnets used, which has placed limitations on freedom of the speaker design. Moreover, the problem has also existed that, because a coil is provided for each bar magnet along the longitudinal direction of the bar magnets, there has been a lack of flexibility in the setting of the speaker impedance to a suitable value.
In addition, in a conventional flat speaker, because the distance from the vibrating membrane to the magnetic pole face of the bar magnets is different by the thickness of the bar magnets from the distance from the vibrating membrane to the portions of the yoke between the bar magnets, a phase difference is caused in the sound generated from the vibrating membrane which is reflected from the rear magnetic pole face and the yoke so as to reach the vibrating membrane surface. Therefore, the further problem exists that, because the vibrating membrane is twisted due to the sound pressure distribution corresponding to this phase difference becoming a noise component in the acoustic signal, the sound quality is further deteriorated.
In order to solve these problems, the filling of the space between the vibrating membrane and the magnetic pole face with a pliable material such as sponge or the like can be considered. However, the vibration of the vibrating membrane is hindered by the pliable material so that the sound quality of low sounds in particular is deteriorated.
Because a plurality of bar magnets are disposed such that the longitudinal directions thereof are parallel, the lengths of the portions interlinking with the magnetic fields of the respective coils are about twice the product of the long side of the bar magnet and the number of windings of the coil. The proportion of the surface area occupied thereby with respect to the surface area of the vibrating membrane at the portion interlinking with the magnetic field of the coil is low. As a result, the efficiency of acoustic conversion is poor and a sufficient sound volume cannot be obtained, and in addition, sufficient sound quality cannot be obtained.
Further, the configuration of the speaker is determined by the length of the bar magnet and the number of bar magnets which are provided, such that the degrees in freedom in designing the configuration of the speaker are limited. Moreover, a coil is provided for each bar magnet along the longitudinal direction of the bar magnet. Thus, a problem arises in that there is little flexibility in setting the impedance of the speaker to an appropriate value.
Moreover, although, in conventional flat speakers, the vibrating membrane is placed adjacent to the magnetic pole face of the bar magnets, a gap exists between the vibrating membrane and the magnetic pole face. Therefore, the problem exists that the flat speaker itself is made too thick.
Furthermore, if the shape of a conventional flat speaker is made still larger and narrower, slackness develops in the vibrating membrane leading to the vibrating membrane and the yoke no longer being parallel to each other. Consequently, the distances from each point on the vibrating membrane to the yoke or the magnetic pole face of the bar magnets is not uniform. The result of this is that the drawback arises that a phase difference is generated in the reflected sound which is reflected by the yoke and the like so as to come back to the vibrating membrane, and twisting of the vibrating membrane is caused by the sound pressure distribution. This creates noise and a reduction in sound quality.
The present invention has been achieved in order to solve the above problems in the prior art and the first object of the present invention is to provide a flat acoustic conversion device in which twisting of the vibrating membrane is reduced and noise components are decreased.
The second object of the present invention is to provide a flat acoustic conversion apparatus in which the length of the portion of the coils intersecting the magnetic field is lengthened thus increasing the proportion of the surface of the vibrating membrane occupied by the coils and improving the acoustic conversion efficiency. This, in turn, leads to an improvement in sound quality.
The third object of the present invention is to provide a flat acoustic conversion device in which the lengths of the portions interlinking with the magnetic fields of the coils are long and the proportion of the surface area occupied by the coils on the surface of a vibrating membrane is increased, such that the efficiency of acoustic conversion is improved and sound quality is improved.
The fourth object of the present invention is to provide a flat acoustic conversion device which is even thinner than the prior art.
The fifth object of the present invention is to provide a flat speaker device which can always provide a high quality audio output regardless of the shape of the vibrating membrane.